(1) Field of the Invention
This invention relates to an impact-resistant methacrylic resin composition excellent in surface appearance and transparency.
(2) Description of the Prior Art
A methacrylic polymer resin composed mainly of methyl methacrylate has such characteristics as a beautiful appearance, excellent transparency, high weather resistance and good fabricability and is widely used in various fields as cast sheets, extrusion-molded articles and injection-molded articles. However, this resin is hard and brittle. Several methods have heretofore been proposed to eliminate this defect, but no method for eliminating the brittleness without sacrificing the characteristics of the methacrylic resin has yet been realized.
The most popular and effective method for overcoming the defect of brittleness of a methacrylic resin and imparting an impact resistance thereto comprises dispersing particles of an elastomer, which is rubbery at normal temperature, discontinuously in a continuous phase of a methacrylic resin. As the rubbery elastomer, there are used an unsaturated rubbery elastomer composed mainly of butadiene, and a saturated rubbery elastomer, such as an acrylic acid ester polymer, composed mainly of butyl acrylate, 2-ethylhexyl acrylate or other acrylates and an ethylene/vinyl acetate copolymer. An unsaturated rubber elastomer has the problem of poor weather resistance due to unsaturated double bonds present in the main polymer chain. A saturated rubbery elastomer has the problems of, for example, poor modulus of elasticity and elastic recovery of the rubber component per se and poor graft-polymerizability with a hard resin component, therefore insufficient impact resistance and degraded transparency and surface gloss. Furthermore, the surface appearance is not satisfactory because of flow mark.
Important factors in preparing an impact-resistant resin composition of the above-mentioned two-component type comprising a discontinuous phase of rubbery elastomer particles homogeneously dispersed in a continuous phase of a hard resin such as a methacrylic resin are the particle size of the rubbery elastomer, the crosslinking degree of the rubber elastomer, the graft polymerizability of the hard resin phase to the rubber phase and the molecular weight of the hard resin phase. In fact, the relative superiority and balance of the resin characteristics are greatly influenced by these factors. Among these factors, the crosslinking degree of the rubber phase, the graft polymerizability of the hard resin to the rubber phase and the molecular weight of the hard resin phase are especially important.
As the crosslinking degree of the rubbery elastomer is high, the surface appearance characteristics such as the surface gloss and prevention of flow marks are improved, but the impact resistance is degraded.
The graft polymerizability of the hard resin phase to the rubber phase greatly influences the compatibility and dispersibility of the rubbery elastomer with the continuous resin phase. The impact resistance, transparency, stress-whitening resistance, surface gloss and flow processability are greatly influenced by this graft polymerizability. When a saturated rubbery elastomer is used, the graft polymerizability is ordinarily low, and special care should be taken. It has recently been proposed that a specific polyfunctional monomer called a "copolymerizable grafting monomer" be used in the polymerization for formation of an acrylic elastomer. This monomer, however, is used as a crosslinking agent in a broad sense and no complete improvement can be obtained.
A higher molecular weight of the hard resin phase is effective for improving the impact resistance but causes degradation of surface appearance and fabricability.
As is apparent from the foregoing description, the factors behave independently, and it is very difficult to set the general properties of an impact-resistant resin composition with a good balance. Thus, an impact-resistant methacrylic resin comparable to an unmodified methacrylic resin in transparency, surface appearance, and fabricability has not been developed.
In connection with an impact-resistant resin composition or impact-resistant methacrylic resin composition comprising an acrylic acid ester elastomer as the rubber phase, which is excellent in weather resistance, there has recently been proposed a method in which a hard resin is included in the interiors of rubber particles so as to improve the impact resistance-manifesting effect of the rubber phase, the transparency and stress-whitening resistance of the shaped article, and the pearl-like gloss due to deformation of the rubber particles (see Japanese Examined Patent Publication No. 52-30996, Japanese Unexamined Patent Publication No. 48-55233, U.S. Pat. No. 3,661,994 and U.S. Pat. No. 3,793,402). Indeed, the intended effects can be attained to some extent according to this method, but the resulting resin composition is still inferior to an unmodified methacrylic resin in transparency and surface appearance.